1. Field of the Invention
The present invention relates generally to audio/video (AV) systems, and more specifically, to a synchronizing loop for providing a low-jitter synchronized clock signal that is further synchronized to high-level events.
2. Background of the Invention
Digital audio systems are prevalent in the areas of home entertainment, professional production of multimedia and computer reproduction and generation of multimedia sources. Increasingly, digital audio data is transported from sub-system to sub-system and device to device in both consumer and professional installations. The digital audio data is generally provided with, or has embedded, a timing reference that has too much jitter to act as a stable sample clock reference. Furthermore, the timing information may be provided at a frequency that is lower than the clock rate needed to operate a digital audio sink, such as a digital-to-analog converter (DAC) that receives the incoming digital audio data stream.
Therefore, clock generation circuits that provide stable clock references synchronized to a digital audio stream's timing information are frequently required. The clock generating circuits must generally provide a very low-jitter clock from a timing reference that may have a large amount of jitter present and/or from an additional interface clock that also may have a large amount of jitter.
World Intellectual Property Organization (WIPO) International Publication WO2004/088845A1 entitled “METHOD OF ESTABLISHING AN OSCILLATOR SIGNAL”, filed by Christopher Julian Travis on Oct. 14, 2004, discloses such clock synchronizing circuits. A clock generated by an analog phase-lock loop (APLL) is synchronized to incoming time information by using a numerically-controlled oscillator that is controlled by a phase-frequency detector that compares the incoming timing information with a division of the output clock to generate an intermediate clock signal. However, the intermediate clock signal frequency places a limit on the performance of the synchronizing circuit, since the intermediate clock signal frequency typically cannot exceed the divided output clock used for phase comparison in the APLL. Furthermore, the effective information update rate in the phase comparator of the APLL is limited to a single bit of information at the intermediate clock signal frequency. The result is that the loop bandwidth of the APLL must be low enough to remove jitter from the intermediate clock signal, but high enough to remove the inherent noise of the APLL's oscillator.
The constraints on the above-described synchronization circuit reduce applicability of the circuit and place limitations on the jitter-reduction performance obtainable for a given application. The above-incorporated Parent U.S. Parent Application discloses and claims solutions that utilize numerically controlled oscillators to provide the desired jitter reduction. However, in some applications, it is further necessary to provide synchronization to high-level events, such as when synchronizing audio streams to video lines or frames.
Therefore, it would be desirable to provide an improved synchronization circuit and method for generating a low-jitter clock source in synchronization with a jittery timing reference that further incorporates high-level event synchronization.